Inlets for aircraft nacelles, wing leading edges, horizontal stabilizers, vertical fins, and other aircraft components may be subject to ice build-up during flight. A heat source may heat the components to prevent the ice build-up or to remove ice after it has built up. The heat source most commonly used today is hot bleed air from a gas turbine engine that heats the backside of the external surface subject to ice build-up. Electric resistance heating has also been proposed and is entering service in a small number of applications.
A pressure regulating valve is commonly used to manage the flow rate of bleed air supplied to the deicing system. The pressure regulating valve regulates the pressure of air leaving the valve and flowing toward a cavity formed behind the surface subject to icing. By regulating the outlet pressure, and with constant duct and outlet geometry, the volumetric flow rate of air into the de-icing system is relatively constant. However, the pressure regulating valve does not account for changes in the density and temperature of the bleed air, and changes in the ambient density and temperature. The temperature of the bleed air from the compressor varies based on engine operating conditions. For example, during takeoff, the engine is operating at high throttle, and the bleed air is at a relatively high temperature. Thus, even though the volumetric flow rate is managed, the heat rate and the cooling rate are variable, and therefore the temperature of the heated aircraft surface and associated structure varies widely. The material and design of the heated surface and associated structure must be designed to withstand the possibility of the maximum temperature (which is usually heavy aircraft take-off on a hot, dry ambient day at sea-level, even though this condition may occur infrequently. This condition often drives the design of the heated components, resulting in heavier structures to withstand thermal expansion and/or more expensive materials resistant to the heat.
Although managing the de-icing system by regulating the pressure of the heated air has its advantages, such as simplicity and reliability of the valve components, it may sometimes result in penalties paid due to the heavier and more expensive structures needed to withstand maximum temperatures. It would be advantageous to have more control, or another element of control, over the heating and cooling rate, in order to minimize the maximum temperatures. This extra element of control would be especially beneficial if it can also operate as a backup to the pressure regulating valve when it fails, to help manage the maximum temperatures in that condition.